Field of the Invention
The present invention relates to a measuring technique to measure a wavefront aberration of an object to be measured, such as an optical element.
Description of the Related Art
As methods of measuring a wavefront of light transmitted through or reflected by an optical element as an object to be measured (hereinafter simply referred to as “an object”), measuring methods using a Talbot interferometer and a Shack-Hartmann sensor are conventionally known. Calculating a difference between a measurement result obtained by one of these measuring methods and a design value of aberration of the optical element enables measuring a wavefront aberration corresponding to the aberration of the optical element.
Japanese Patent Laid-Open No. 2010-151578 discloses a method of measuring aberration of an object from a difference (that is, a wavefront aberration) between a measured value of a wavefront measured by the Talbot interferometer or the Shack-Hartmann sensor and a value of a reference wavefront. Moreover, it discloses a method of calculating the reference wavefront by using an optical path length calculated by using a reference object whose shape and refractive index distribution are known.
However, the reference wavefront calculated by using the optical path length has, if the wavefront does not have a planar shape, a difference from the measured value of the wavefront measured by the Talbot interferometer or the Shack-Hartmann sensor. This difference can be easily explained, if the wavefront is a spherical wavefront, by using numerical expressions. For example, an optical path length Ws shown in FIG. 10 from a point light source to a sensor surface which is a planar surface can be expressed by following expression (1) where L represents a distance from the point light source to the sensor surface and r represents a distance (radius) from a center of the sensor surface:
                    Ws        =                                                                              L                  2                                -                                  r                  2                                                      -            L                    ∼                                                    r                2                                            2                ⁢                                                                  ⁢                L                                      -                                          r                4                                            8                ⁢                                                                  ⁢                                  L                  3                                                      +                                          r                6                                            16                ⁢                                                                  ⁢                                  L                  5                                                                                        (        1        )            
On the other hand, when measurements of a wavefront of light from a same object by using the Talbot interferometer and the Shack-Hartmann sensor are made, stripes or bright points at equal intervals are measured in both the measurements. Since a differential wavefront which is a differential value of this measured wavefront is a tilted wavefront, the measured wavefront Wm is expressed by the following quadratic function of r (expression (2)):
                    Wm        =                              r            2                                2            ⁢                                                  ⁢            L                                              (        2        )            
The optical path length Ws expressed by expression (1) and the measured wavefront Wm expressed by expression (2) coincide with each other only when the distance L is infinite, that is, light from the object is collimated light, and on the other hand have a difference from each other when the distance L is not infinite.
Such a difference is difficult to be expressed by numerical expressions when the measured wavefront is an aspheric surface, so that the difference cannot be easily corrected. Therefore, when calculating the reference wavefront by using the light path length, there may be a case where an accurate difference between the measured wavefront and the reference wavefront cannot be obtained, which is a problem that measurement accuracy of the wavefront aberration decreases.